first_value

CREATE TABLE test_data
(
    a Int64,
    b Nullable(Int64)
)
ENGINE = Memory;

INSERT INTO test_data (a, b) VALUES (1,null), (2,3), (4, 5), (6,null);

SELECT first_value(b) FROM test_data;

┌─any(b)─┐
│      3 │
└────────┘

SELECT first_value(b) ignore nulls FROM test_data

┌─any(b) IGNORE NULLS ─┐
│                    3 │
└──────────────────────┘

SELECT first_value(b) respect nulls FROM test_data

┌─any(b) RESPECT NULLS ─┐
│                  ᴺᵁᴸᴸ │
└───────────────────────┘

SELECT
    first_value_respect_nulls(b),
    first_value(b)
FROM
(
    SELECT *
    FROM test_data
    ORDER BY a ASC
)

┌─any_respect_nulls(b)─┬─any(b)─┐
│                 ᴺᵁᴸᴸ │      3 │
└──────────────────────┴────────┘